dt-bindings: Add doc for FriendlyARM NanoPi R2C

Add devicetree binding documentation for the FriendlyARM NanoPi R2C.
Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20230325074022.9818-2-cnsztl@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -185,9 +185,11 @@ properties:
           - const: firefly,rk3566-roc-pc
           - const: rockchip,rk3566
 
-      - description: FriendlyElec NanoPi R2S
+      - description: FriendlyElec NanoPi R2 series boards
         items:
-          - const: friendlyarm,nanopi-r2s
+          - enum:
+              - friendlyarm,nanopi-r2c
+              - friendlyarm,nanopi-r2s
           - const: rockchip,rk3328
 
       - description: FriendlyElec NanoPi4 series boards